In a continuation of studies on the dynamics of phosphate metabolism we have been investigating the properties, activities and isozyme composition of inorganic pyrophosphatase (EC 3.6.1.1) in normal and neoplastic tissues of humans and various animals. The major enzyme from rat liver and a rat hepatoma has been purified to homogeneity, and the human placental enzyme is now being purified and characterized. Four isozymes have been identified by agarose gel electrophoresis; three in the cytosol and one and possibly two in the mitochondria.